The recording density of optical storage devices such as magneto-optical disks depends greatly on the size of a light beam converged as a light spot on a recording medium in recording and reproduction. A recently proposed system enables the reproduction of a bit which is smaller than the spot size of a light beam. In optical recording, the light beam is usually converged to a diffraction limit by a converging lens. This causes the intensity distribution of light to be Gaussian and the distribution of temperature on the recording medium to become substantially Gaussian. Consequently, a portion which has been exposed to light and heated to a temperature above a certain temperature has a size smaller than the spot size of the light beam. If it is possible to reproduce only a portion having a temperature above the certain temperature, the recording density can be significantly increased.
With reference to FIG. 6, the following description discusses the system for reproducing a bit smaller than the spot size of a light beam, recorded on a magneto-optical disk (disclosed in the U.S. patent application Ser. No. 07/870,630 now abandoned).
The magneto-optical disk is composed chiefly of a transparent substrate 25, and a readout layer 26 and a recording layer 27 formed thereon. The Curie temperature for the readout layer 26 is considerably higher than that for the recording layer 27. Another characteristic of the readout layer 26 is that it exhibits in-plane magnetization at room temperature and perpendicular magnetization when its temperature becomes higher than a certain temperature as a result of the application of the light beam.
In reproduction, when the light beam is applied to the readout layer 26, the temperature distribution at a region exposed to the light beam becomes Gaussian. Consequently, only a portion, which corresponds to the central portion of the light beam and is thus smaller than the diameter of the light spot, is heated to a temperature over the certain temperature. With the rise of the temperature, there is a change from in-plane magnetization to perpendicular magnetization in the readout layer 26. At this time, the magnetizing direction in the recording layer 27 is copied to the readout layer 26 by the exchange coupling force between the readout layer 26 and the recording layer 27.
As a result, magneto-optical effect occurs only at the portion which has been heated to above the certain temperature and a change from in-plane magnetization to perpendicular magnetization is observed. And, information recorded on the recording layer 27 is reproduced using reflected light from the portion.
When the light beam moves to reproduce the next recorded bit, the temperature of the previously reproduced portion decreases and the magnetization of the readout layer 26 changes from perpendicular magnetization to in-plane magnetization. Since the portion whose temperature has dropped below the certain temperature no longer exhibits the magneto-optical effect, the information recorded in the portion of the recording layer 27 is masked by the in-plane magnetization of the readout layer 26, preventing reading of the information. It is thus possible to reproduce only a desired bit without having interference between signals from the desired bit and adjacent bits, preventing noise.
As described above, only a portion whose temperature has risen over a certain temperature is reproduced, it is possible to reproduce a recorded bit smaller than the diameter of the light spot, improving the recording density.
With regard to methods of controlling the rotation of a magneto-optical disk in a magneto-optical recording and reproducing device, they are generally classified into two types, namely the CLV (constant linear velocity) method and the CAV (constant angular velocity) method. With the CLV, the disk uses a constant linear velocity of track relative to pickup so the rotational speed is a function of the radius of the track, and varies as the pickup moves across the disk. The CLV is obtained by changing the rotational speed of a motor which rotates a disk. On the other hand, with the CAV method, the motor always rotates at a uniform speed. Consequently, the linear velocity of track relative to the pickup is a function of the radius of the track. Advantage of the CAV over the CLV is that the structure of a motor control system is simplified because the rotational speed of the motor is uniform.
However, with the CAV, a portion to be exposed to the light beam and heated to a temperature over a certain temperature has different sizes in the central area and the peripheral area of the magneto-optical disk. Specifically, since the linear velocity of the disk at the central area is lower compared to that at the peripheral area, when the intensity of the light beam applied to the central area is the same as the intensity of the light beam applied to the peripheral area, the size of a portion heated to a temperature over the certain temperature in the central area becomes larger than that in the peripheral area. Therefore, in the recording medium shown in FIG. 6, the size of a portion on the readout layer 26 where a change from in-plane magnetization to perpendicular magnetization is observed becomes larger toward the center of the disk. As a result of a portion includes not only a desired bit but also adjacent bits, causing noise in reproduction.
To avoid such a problem, the intensity of light beam may be changed as a function of the radius of the disk. However, this method is not practically desirable because a burden of the light beam control system increases.